Load switch circuit and control method

ABSTRACT

A load switch circuit includes a power transistor, the first terminal is configured to receive the power supply voltage, and the second terminal is the output terminal of the load switch circuit and is coupled with an external inductive load; a clamping module including at least a mutually coupled clamping unit and a driving unit; the clamping unit, including a voltage-current converter and a first resistor, the first resistor is coupled between the output terminal of the voltage-current converter and the second terminal of the power transistor, the output terminal of the drive unit is coupled to the control terminal of the power transistor when the difference between the power supply voltage and the output voltage of the power transistor is greater than or equal to the preset clamping threshold, and the clamping unit outputs an effective drive control signal to the driving unit.

BACKGROUND Technical Field

This application relates to the field of electrical control, and inparticular relates to a load switch circuit and a control methodthereof.

Description of the Related Art

Load switch circuits are widely used in high-voltage equipment such asautomotive electronics and gas water heaters. When driving a relativelylarge inductive load, after the switch is turned off, it is necessary toprovide freewheeling path for inductive current. FIG. 1 is a schematicdiagram of an existing circuit that uses a switch external circuitstructure to realize inductive freewheeling. After the power transistoris turned off, current continues to flow through the inductor, and thepotential at the output node is pulled down to a very large negativepotential, which may cause the power transistor to break down. Afreewheeling diode D with the anode coupled to ground potential and thecathode coupled to the external load inductance prevents the output nodepotential from being severely pulled down. However, since the voltagedrop on the forwarded freewheeling diode is only about 0.7V, thedischarge speed of the inductive load current is relatively slow.

FIG. 2 is a schematic diagram of an existing circuit that utilizes theinternal circuit structure of a load switch to implement inductivefreewheeling. The circuit provides a mechanism for clamping the outputvoltage inside the load switch circuit. However, as shown in the figure,clamping the output voltage of the load switch circuit is achieved byone or more diodes connected in series. Therefore, the smallest unit foradjusting the clamping voltage in this mechanism is the breakdownvoltage of a single diode, such as 5V.

BRIEF SUMMARY

The present application provides a load switch circuit including a powertransistor and a clamping module. A first terminal of the powertransistor is configured to receive the power supply voltage, and asecond terminal is the output terminal of the load switch circuit and iscoupled to an external inductive load. The clamping module includes atleast a clamping unit and a driving unit coupled together. The clampingunit includes a voltage-current converter and a first resistor, and thefirst resistor is coupled to the output terminal of the voltage-currentconverter and the power transistor. The positive input terminal of thevoltage-current converter is configured to receive the power supplyvoltage, and the negative input terminal is coupled to the secondterminal of the power transistor. The current output of thevoltage-current converter generates a reference voltage drop on thefirst resistor. The output terminal of the driving unit is coupled tothe control electrode of the power transistor. When the differencebetween the power supply voltage and the output voltage of the powertransistor is greater than or equal to the preset clamping threshold,the clamping unit is configured to output an effective driving controlsignal to the driving unit, and the driving unit outputs an effectivedriving signal to turn on the power transistor. The preset clampingthreshold is the sum of the reference voltage drop and the thresholdvoltage of the first transistor.

In some embodiments, the proposed load switch circuit further includes adriving unit comprising a first transistor. The control electrode of thefirst transistor is coupled to the output terminal of thevoltage-current converter, the second electrode is coupled to thecontrol electrode of the power transistor, and the first electrode ofwhich is configured to receive the power supply voltage. A firstresistive branch is coupled between the second electrode of the firsttransistor and the second electrode of the power transistor, and thetype of the first transistor is complementary to the type of powertransistor.

In some embodiments, the proposed clamping module further comprises atrigger unit configured to receive the output voltage of the powertransistor. When the output voltage is lower than the ground potential,the power supply voltage is supplied to the clamping unit.

In some embodiments, the proposed clamping unit further comprises afirst diode (ZD3) and a second transistor (MP2), wherein the cathode ofthe first diode (ZD3) is configured to receive the power supply voltage,the anode of the first diode (ZD3) is coupled to the control electrodeof the second transistor (MP2), the first electrode of the secondtransistor (MP2) is coupled to the second electrode of the powertransistor, and the second transistor (MP2) is coupled to the negativeinput terminal of the voltage-current converter; and the positive inputterminal of the voltage-current converter is configured to receive thepower supply voltage, wherein a second resistive branch is coupledbetween the control electrode of the second transistor (MP2) and thesecond terminal of the power transistor (MN3), and the type of secondtransistor (MP2) is of the same type as the first transistor (MP4).

In some embodiments, the proposed second resistive branch includes asecond resistor (R3) coupled between the control electrode of the secondtransistor (MP2) and the second terminal of the power transistor (MN3).

In some embodiments, the proposed clamping unit further comprises athird transistor (MP3), the control electrode of which is coupled to thecontrol electrode of the second transistor (MP2), and the first resistor(R4) is coupled between the first electrode of the third transistor(MP3) and the second electrode of the power transistor (MN3), and thesecond electrode of the third transistor (MP3) is coupled to the outputterminal of the voltage-current converter, wherein the type of the thirdtransistor (MP3) is the same as the type of the second transistor (MP2).

In some embodiments, the proposed trigger unit comprises a fourthtransistor (MN2), a fifth transistor (MN1) and a sixth transistor (MP1)wherein the control electrode of the fourth transistor (MN2) isconfigured to receive a clamp control signal, the first electrode ofwhich is coupled to the control electrode of the fifth transistor (MN1),and the second electrode is configured to receive ground potential; thefirst electrode of the fifth transistor (MN1) is coupled to the controlelectrode of the sixth transistor (MP1), and the second electrode iscoupled to the second electrode of the power transistor (MN3), a thirdresistive branch is coupled between the fifth transistor (MN1) and thesecond electrode of the power transistor (MN3); the first electrode ofthe sixth transistor (MP1) is coupled to the positive input terminal ofthe voltage-current converter, and the second electrode is configured toreceive the power supply voltage.

In some embodiments, the proposed trigger unit further comprises asecond diode (ZD1), the cathode of which is configured to receive thepower supply voltage, and the anode of which is coupled to the controlelectrode of the sixth transistor (MP1).

In some embodiments, the proposed trigger unit further comprises a thirdresistor (R1) coupled between the control electrode and the secondelectrode of the sixth transistor (MP1).

In some embodiments, the proposed trigger unit further comprises a thirddiode (ZD2) whose cathode is coupled to the control electrode of thefifth transistor (MN1), and whose anode is coupled to the secondelectrode of the fifth transistor (MN1).

In some embodiments, the proposed third resistive branch includes afourth resistor (R2) coupled between the second electrode of the fifthtransistor and the second electrode of the power transistor in between.

In some embodiments, the proposed first resistive branch includes afifth resistor (R5) coupled between the control electrode and the secondelectrode of the power transistor (MN3).

The present application further proposes an electronic device comprisingthe load switch circuit according to the previous description.

The present application further proposes a control method of a switchcircuit, including setting the equivalent resistance of thevoltage-current converter inside the switch circuit to generate areference current; generating a preset clamping threshold based on atleast the reference voltage generated by the reference current on thereference resistor; and when the difference between the power supplyvoltage and the output voltage of the switching circuit is greater thanor equal to the preset clamping threshold, the power transistor coupledto the load inside the switching circuit is turned on, thereby providinga freewheeling path for the inductive load current.

In some embodiments, the proposed control method further comprises whenthe output voltage of the switch circuit is lower than the ground level,the power supply voltage is provided to the voltage-current converter.

The proposed load switch circuit in the present application provides aclamping module located inside the switch circuit and a freewheelingpath for the inductive load current through the power transistors in theswitch circuit. Furthermore, a voltage-current converter is used togenerate the reference voltage on the reference resistor to form theclamp threshold, this is a simple flexible mechanism for the adjustmentof the clamp threshold. In addition, this application also provides amechanism to automatically trigger the clamp function when the outputvoltage of the switch circuit is lower than the ground level, so thatthe clamp module is in a power-off state when the output voltage is notpulled to a negative value, this reduces the overall power consumptionof the circuit.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The embodiments are shown and explained with reference to the drawings.These drawings are used to clarify the basic principle, so that onlyaspects necessary for understanding the basic principle are shown. Thedrawings are not to scale. In the drawings, the same reference numeralsindicate similar features.

FIG. 1 is a schematic diagram of an existing circuit that uses a switchexternal circuit structure to realize inductive current freewheeling.

FIG. 2 is a schematic diagram of an existing circuit that uses theinternal circuit structure of a load switch to implement inductivecurrent freewheeling.

FIG. 3 is a schematic diagram of a load switch module according to someembodiments of the present application.

FIG. 4 is a schematic diagram of a partial module of a load switchaccording to some embodiments of the present application.

FIG. 5 is a schematic partial circuit diagram of a load switch accordingto some embodiments of the present application.

FIG. 6 shows a working sequence diagram of a load switch according tosome embodiments of the present application.

DETAILED DESCRIPTION

In the following detailed description of the preferred embodiments,reference will be made to the accompanying drawings that constitute apart of this application. The attached drawings show specificembodiments that can implement the present application by way ofexample. The exemplary embodiments are not intended to be exhaustive ofall embodiments according to this application. It can be understood thatwithout departing from the scope of the present application, otherembodiments can be used, and structural or logical modifications canalso be made. Therefore, the following detailed description is notrestrictive, and the scope of the present application is defined by theappended claims.

The technologies, methods, and equipment known to those of ordinaryskill in the relevant fields may not be discussed in detail, but whereappropriate, the technologies, methods, and equipment should be regardedas part of the specification. The connection between the units in thedrawings is only for convenience of description, which means that atleast the units at both ends of the connection communicate with eachother, and it is not intended to limit the communication betweenunconnected units.

In the following detailed description, reference may be made to thedrawings of each specification used as a part of this application toillustrate specific embodiments of the application. In the drawings,similar reference numerals describe substantially similar components indifferent drawings. Each specific embodiment of the present applicationis described in sufficient detail below, so that a person of ordinaryskill with relevant knowledge and technology in the field can implementthe technical solution of the present application. It should beunderstood that other embodiments can also be used or structural,logical or electrical changes can be made to the embodiments of thisapplication.

The transistor in this application, if not specifically indicatedotherwise, may be an NMOS or PMOS transistor, and may include a controlelectrode, a first electrode (drain or source), and a second electrode(source or drain). The drain and source of the transistor in theembodiments of the present application can be exchanged according to thetransistor type or the bias state. Other suitable transistors, such asNPN or PNP bipolar junction transistors, may also be used and areincluded in the scope of the disclosure.

The following defines the high voltage level as the asserted logic stateand the low voltage level as the de-asserted logic state. Of course, theuse of complementary transistors to replace the circuits formed in theembodiments herein also belongs to the protection scope of the presentapplication.

This application proposes a load switch circuit, in which the circuitsthat clamp the output voltage are located inside the load switchcircuit, and the clamp voltage can be flexibly changed or adjusted.

FIG. 3 is a block diagram of a load switch according to embodiments ofthe present application. As shown in the figure, the load switch 30 mayinclude a switch control module 301, configured to receive an inputsignal IN for controlling the state of the switch, and control the powertransistor 303 to turn on or off based on at least the input signal IN.The power switch 30 may also include an output clamp module 302,configured to receive the output signal VOUT of the power transistor 303and activate the clamping function when VOUT reaches a preset clampingthreshold to limit the drain-source voltage difference of the powertransistor 303. As a result, the output signal VOUT of the powertransistor 303 is prevented from pulling down to a level that causes thepower transistor 303 to break down. The load switch 30 may also includea power transistor 303 configured to realize the function of a powerswitch.

FIG. 4 is a schematic diagram of a portion of a load switch according tosome embodiments of the present application. As shown in the figure, theclamping module 302 may include a trigger unit 3021, a clamping unit3022, and a driving unit 3023. Among them, the trigger unit 3021 isconfigured to receive the output voltage VOUT of the power transistor303, and is triggered to start when VOUT turns into negative, andprovides the power supply voltage VS to the clamping unit 3022. Theclamping unit 3022 outputs a driving control signal to the drivingmodule 3023 when VOUT is pulled down below a preset threshold. The drivemodule 3023 outputs a clamping signal under the action of the drivecontrol signal, so that the power transistor 303 is turned on, assiststhe load inductance L to release current, and clamps the output voltageto a fixed potential. As the current in the load inductance is graduallyreduced, the output voltage VOUT will return to the ground level.

FIG. 5 is a schematic diagram of a portion of a load switch according tosome embodiments of the present application. As shown in the figure, theload switch may include, for example, an N-type power transistor MN3.FIG. 5 shows details of the clamp module 302, and the switch controlmodule 301 is not shown in FIG. 5 for simplicity purposes. Those skilledin the art know that the circuit described below is only one exampleembodiment, and the scope of the present application is not limited tothis example embodiment. Those skilled in the art also know that acircuit obtained by merely changing the type of MOS transistor, e.g., ina complementary manner, is still within the scope of this application.

According to some embodiments, the trigger unit 3021 may include, forexample, an N-type transistor MN1, and an N-type transistor MN2. Thegate of MN2 is configured to receive the clamp module control signalEN_VDS_CLAMP. This clamp module control signal is valid only when thepower transistor 303 is in the off or non-working state, and the entireclamp module is activated when EN_VDS_CLAMP is valid.

According to some embodiments, the drain of MN2 is coupled to the gateof transistor MN1, and its source is configured to receive a low voltagepotential, e.g., the ground potential. The source of the transistor MN1is coupled to the source of the power transistor 303 and is configuredto receive the output voltage VOUT of the power switch. According tosome embodiments, the trigger unit 3021 may further include a resistorR2 coupled between the source of the transistor MN1 and the outputterminal of the power transistor, which functions to prevent thetransistor MN1 from being burnt down by an excessive gate-sourcevoltage. Of course, other structures can also be used to form thisresistive branch.

According to some embodiments, the trigger unit 3021 may further includea P-type transistor MP1, the source of which is configured to receivethe power supply voltage VS, and the drain of which is coupled to theclamping unit 3022 for providing the clamping unit with the powervoltage VS_INT. According to some embodiments, the trigger unit 3021 mayfurther include a Zener diode ZD1. The anode of the Zener diode ZD1 iscoupled to the gate of the transistor MP1, and the cathode of the Zenerdiode ZD1 is coupled to the source of the transistor MP1. The triggerunit 3021 may also include a resistor R1 connected in parallel with ZD1.

According to some embodiments, when EN_VDS_CLAMP is at a valid level,e.g., a high level, for example, the transistor MN2 is turned on, theclamp module is activated, and the gate of the transistor MN1 isconfigured to receive the ground level through the turned-on transistorMN2. Therefore, when the load switch 30 is turned off and the inductiveload L pulls down the output voltage VOUT to a negative voltage, thetransistor MN1 is turned on. The turn on of transistor MN1 can providethe output voltage VOUT to the gate of the transistor MP1. In this case,the gate potential of the transistor MP1 is lower than its sourcepotential VS, so the transistor MP1 is turned on. The power supplyvoltage is supplied to the clamping unit 3022 as VS_INT.

According to some embodiments, the reverse breakdown voltage of theZener diode ZD1 may be, for example, 5V. When the voltage differencebetween VOUT and VS is greater than or equal to 5V, the Zener diode ZD1may be broken down to form a conductive path, thereby providingprotection for the transistor MP1. The role of R1 is to ensure that MP1can be turned off completely when MP1 is in the closed state.

Based on the above-mentioned structure and operation mode of the triggerunit 3021, the clamping unit 3022 cannot receive the power supplyvoltage when the output of the load switch is not lower than the lowlevel at the source of N-type transistor MN2, e.g., the ground level, sothis solution reduces the overall power consumption of the clamp module3022. Of course, regardless of power consumption, according to otherembodiments, the trigger unit 3021 can also be used only to receive theclamping module control signal, and regardless of the value of theoutput voltage VOUT of the load switch circuit, the clamping unit 3022may also be always coupled to the supply voltage.

According to some embodiments, the clamping unit 3022 may include avoltage-current converter V-I, a P-type transistor MP2, a Zener diodeZD3, and a resistor R3. The cathode of the Zener diode ZD3 is configuredto receive VS_INT (VS_INT=VS), and its anode is coupled to the gate ofthe transistor MP2. The resistor R3 is coupled between the gate of thetransistor MP2 and the output terminal of the load switch circuit. Thedrain of the transistor MP2 is also coupled to the output terminal ofthe load switch circuit for receiving VOUT. Both the positive inputterminal and a control terminal of the voltage-current converter V-I areconfigured to receive the power supply voltage VS_INT, and the negativeinput terminal and the other control terminal are both coupled to thesource of the transistor MP2.

According to some embodiments, the clamping unit 3022 may furtherinclude a P-type transistor MP3 and a resistor R4. The gate of thetransistor MP3 is coupled to the gate of the transistor MP2, and thesource is coupled to the output terminal of the voltage-currentconverter V-I. The resistor R4 is coupled between the drain of thetransistor MP3 and the output terminal of the load switch circuit.

According to some embodiments, the breakdown voltage of the Zener diodeZD3 may be 5V. When VS_INT−VOUT−VR3 is greater than or equal to 5V,Zener diode ZD3 is broken down, the gate voltage of transistor MP2 isVS_INT−5V, and the source voltage of transistor MP2 is VS_INT−5V+Vsg,where Vsg is the source gate voltage of the transistor MP2 and VR3 is avoltage drop on the resistor R3.

According to some embodiments, the voltage difference between thepositive input terminal and the negative input terminal of thevoltage-current converter V-I may be 5V−Vsg, and the equivalentresistance converted by the voltage-current converter V-I may be Rx.Therefore, the current Il output at the output terminal of thevoltage-current converter V-I can be expressed as (5V−Vsg)/Rx. The usercan adjust the value of Rx as needed to adjust the clamping effect, thatis, the lowest voltage value allowed for VOUT to be pulled down.

Generally, the devices in the voltage-current converter V-I arelow-voltage devices, so the voltage difference at the input terminalsgenerally does not exceed 5V. Since VS and VS_INT are high voltages,such as 40V, the voltage-current converter V-I cannot be directlygrounded to prevent damaging of the voltage devices. In someembodiments, by setting the Zener diode ZD3, the difference of the inputsignal of the voltage-current converter is effectively adjusted towithin 5V. In addition, the source-drain withstand voltage of thetransistor MP2 is much higher than the voltage devices inside thevoltage-current converter V-I, so MP2 provides an appropriate groundingpath for the voltage-current converter V-I.

ZD3, R3, and transistor MP2 are all set to be able to convert the highvoltage signal of the power supply voltage into an acceptable voltagelevel for the voltage-current converter. Other structures can also beused instead of R3 to realize resistive branches. According to differentembodiments, circuits that use other structures to implement thefunction of adjusting the input voltage of the voltage-current converterare also within the protection scope of the present application.

According to some embodiments, when the source gate voltage of thetransistor MP3 is greater than or equal to its threshold voltage, MP3 isturned on, and its source and drain voltages are equal when it is turnedon, and the current Il flows through the resistor R4 and generates avoltage across it. Therefore, either the voltage at the source or theone at the drain of the transistor MP3 can be regarded as VDS_REF+VOUT,where VDS_REF=(5V−Vsg)*R4/Rx. The source voltage VDS_REF+VOUT of thetransistor MP3 is the driving control signal output to the driving unit3023.

According to some embodiments, the driving unit 3023 may include aP-type transistor MP4 and a resistor R5. The source of the transistorMP4 can be configured to receive VS_INT, and its gate is coupled to thesource of the transistor MP3. The resistor R5 is coupled between thedrain of the transistor MP4 and the output terminal of the load switch.When the source to gate voltage (VS_INT)−(VDS_REF)−VOUT of thetransistor MP4 is greater than or equal to the threshold voltage Vth ofthe transistor MP4, the transistor MP4 is turned on.

According to some embodiments, when VOUT drops so that the voltage ofVDS_CLAMP=VS_INT−VOUT exceeds VDS_REF+Vth, the preset clampingthreshold, the transistor MP4 is turned on and the power supply voltageVS_INT is provided to, for example, the gate of the N-type powertransistor MN3. Vth is the threshold voltage of transistor MP4. In thiscase, the power transistor MN3 is turned on, and current flows throughthe power transistor MN3 and the inductive load, thereby assisting theinductor to discharge current. According to some embodiments, the valueof R4 may be in the order of 10 kOhms to 1 MOhm. The value of Rx can becalculated based on the desired VDS_CLAMP.

According to some embodiments, the transistor MP4 is coupled to theoutput terminal of the switch through a resistor R5. Of course, othermethods can be used instead of R5 to form a resistive branch.

After the inductor current is discharged, VOUT is pulled up to 0V, thetransistor MN1 is turned off, the transistor MP1 is also turned off, theclamping unit 3022 does not receive the power supply voltage VS, so itis in no power consumption status.

As the load inductance discharges current, the power transistor MN3 isnot completely turned on as in the normal working state, it only sets acorresponding gate-source voltage according to the magnitude of thecurrent that needs to be discharged, as a result, VOUT is not pulled upto VS_INT. Therefore, there is no confusion with the normal workingstate of MN3. At the same time, due to the reuse of the power transistorMN3, a clamping mechanism inside the load switch is provided, thisoutperforms the solution with external clamping diodes and avoids theinability to dynamically adjust the clamping threshold.

This application provides a solution for realizing a freewheeling pathfor an inductive load by using the internal structure of the loadswitch. In this solution, a voltage-current converter is used togenerate a reference current and then a reference voltage is generatedon the reference resistor, thereby forming a clamping threshold. Byreusing the power transistor, the current of the inductive load isdischarged. Based on this solution, users can flexibly adjust the clampthreshold according to the actual current that needs to be discharged,to achieve precise control of the discharge.

FIG. 6 is a schematic diagram of a timing sequence of a load switchcircuit according to some embodiments of the present application. Asshown in the figure, when the load switch circuit is in the off state,the output voltage VOUT is pulled down to a negative value, but thepull-down amplitude is limited to VDS_CLAMP. This prevents VOUT of theswitching circuit from being pulled down to an excessively negativepotential.

According to some embodiments, the present application also provides acontrol method of a load switch circuit, including setting theequivalent resistance of the voltage-current converter inside the loadswitch circuit to generate a reference current; the reference currentproduces a reference voltage across the reference resistor, thusgenerates a preset clamping threshold; and when the difference betweenthe power supply voltage and the output voltage of the switching circuitis greater than or equal to the preset clamping threshold, the powertransistor is turned on, thereby freewheeling the inductive load.

According to some embodiments, the method further includes providing apower supply voltage to the voltage-current converter when the outputvoltage of the switching circuit is lower than the ground level.

Therefore, although the present application has been described withreference to specific examples, where these specific examples are onlyintended to be exemplary and not to limit the application, it is obviousto those of ordinary skill in the art that without departing from thespirit and scope of the application, the disclosed embodiments may bechanged, added, or deleted.

The disclosure herein provides many different embodiments, or examples,for implementing different features of the described subject matter.Specific examples of components and arrangements are described below tosimplify the present description. These are, of course, merely examplesand are not intended to be limiting. For example, the formation of afirst feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In the description herein, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of thedisclosure. However, one skilled in the art will understand that thedisclosure may be practiced without these specific details. In otherinstances, well-known structures associated with electronic componentsand fabrication techniques have not been described in detail to avoidunnecessarily obscuring the descriptions of the embodiments of thepresent disclosure.

Unless the context requires otherwise, throughout the specification andclaims that follow, the word “comprise” and variations thereof, such as“comprises” and “comprising,” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.”

The use of ordinals such as first, second and third does not necessarilyimply a ranked sense of order, but rather may only distinguish betweenmultiple instances of an act or structure.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its sense including “and/or” unless the contentclearly dictates otherwise.

The various embodiments described above can be combined to providefurther embodiments. Aspects of the embodiments can be modified toprovide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

The invention claimed is:
 1. A circuit, comprising: a first transistor,a second transistor, and a third transistor, wherein: a first terminalof the first transistor is configured to be coupled to a load, a secondterminal of the first transistor is coupled to a gate terminal of thesecond transistor, and a gate terminal of the first transistor iscoupled to a first voltage terminal in response to the third transistorbeing in an off state; a first terminal of the second transistor iscoupled to a second voltage terminal, a second terminal of the secondtransistor is coupled to a gate terminal of the third transistor; and afirst terminal of the third transistor is coupled to the second voltageterminal, a second terminal of the third transistor is coupled to theload.
 2. The circuit of claim 1, comprising a Zener diode coupledbetween the first terminal of the first transistor and the gate terminalof the first transistor.
 3. The circuit of claim 1, comprising a fourthtransistor coupled between the gate terminal of the first transistor andthe first voltage terminal, the fourth transistor controlled by a signalthat is active in response to the third transistor being in the offstate.
 4. The circuit of claim 1, comprising a first resistive unit anda second Zener diode coupled in series between the second terminal ofthe second transistor and the load.
 5. The circuit of claim 4,comprising a voltage-current converter, a first input terminal of thevoltage-current converter coupled to the second terminal of the secondtransistor, a second input terminal of the voltage current convertercoupled to a node point intermediate between the second Zener diode andthe first resistive unit.
 6. The circuit of claim 5, comprising a fifthtransistor, a first terminal of the fifth transistor coupled to thesecond input terminal of the voltage-current converter, a secondterminal of the fifth transistor coupled to the load, and a gateterminal of the fifth transistor coupled to the node point intermediatebetween the second Zener diode and the first resistive unit.
 7. Thecircuit of claim 5, comprising: a first current path coupled to anoutput terminal of the voltage-current converter; and a second currentpath coupled between the second terminal of the second transistor andthe load, wherein: the first current path includes a first voltagedivider circuitry, an output of the first voltage divider circuitryconfigured to control a sixth transistor in the second current path; andthe second current path includes a second voltage divider circuitry, anoutput of the second voltage divider circuitry configured to control thethird transistor.
 8. The circuit of claim 7, wherein the first currentpath includes a seventh transistor, gate terminal of the seventhtransistor coupled to the node point intermediate between the secondZener diode and the first resistive unit.
 9. The circuit of claim 7,wherein the first voltage divider circuitry includes a second resistiveunit, and the second voltage divider circuitry includes a thirdresistive unit.